1. Field of the Invention
The present invention relates to a system for correcting the output from an image sensor for use in an image information reading apparatus employing a solid state array.
2. Description of the Prior Art
Recently a solid state image sensor using larger scale integration techniques, such as a photo diode array, a charge coupled device sensor or the like has been employed as a photoelectric converter in an optical image reading apparatus. However, in using a solid state image sensor having a multiplicity of bits, various problems are encountered. Firstly, the output from the solid state sensor is uneven in magnitude from bit to bit. Secondly, it is difficult to uniformly illuminate an object being read as a matter of practice. Thirdly, the reflectivity is different from object to object. A further problem is that the characteristics of the illumination lamp and the sensor output amplifiers could change with respect to time.
A scheme of interest for correcting the output from the image sensor for eliminating the above described problems are disclosed in Japanese Patent Laying-Open Gazette No. 126,132/1975 and the counterpart U.S. Pat. No. 4,003,021. More specifically, an object being read includes an output correcting portion where no information is recorded, the output correcting portion is scanned several times by a sensor array. The video signal having the best output is selected out of the outputs of the repeated scans and is stored as a correcting signal and whenever portion of the object being scanned contains recorded information, the optical information obtained from the portion being read is compared with a suitable slice level determined by the correcting signal, whereby the comparison output is converted into a logic one level or a logic zero level for transfer thereof as a sensor array output to a subsequent recognition circuit. Although the above described reading scheme provides a level correction of the optical information, the scanning speed of the sensor array is limited to the speed of the analog to digital conversion when converting the sensor output into a video signal to be applied to the recognition circuit, with the result being that high speed scanning is difficult.